Astronomers May Have Discovered 21 Neutron Stars Orbiting Sun-Like Stars

This illustration shows a binary star system consisting of a dense neutron star and a normal Sun-like star (top left). Using data from the European Space Agency's Gaia mission, astronomers have discovered several systems like this one, in which the two bodies are widely separated. Because the bodies in these systems are far apart, on average 300 times the size of the Sun-like star, the neutron star is dormant — it is not actively robbing mass from its companion and is therefore very faint. To find these hidden neutron stars, scientists used Gaia observations to look for wobbles in the Sun-like stars caused by the tugging action of orbiting neutron stars. These are the first neutron stars ever discovered solely through their gravitational influence.

Most stars in our universe come in pairs. Our sun is a solitary one, but many stars like our sun orbit similar stars, while many other exotic pairs between stars and space balls dot the universe. Black holes, for example, often orbit each other. One type of pair that is quite rare is a pair between a sun-like star and a type of extinct star called a neutron star.

Now astronomers led by Karim El-Badry of the California Institute of Technology discovered 21 neutron stars orbiting stars like our Sun. Neutron stars are the dense, burned-out cores of massive stars that have exploded. They are very dim on their own and usually undetectable directly. But when a neutron star orbits a Sun-like star, it tugs on its companion, causing the star to shift back and forth across the sky. Astronomers using the European Space Agency's Gaia mission have captured these noticeable wobbles and discovered a new population of dark neutron stars.

“Gaia is constantly scanning the sky and measuring the oscillations of more than a billion stars, so the chances of finding even very rare objects are high,” says El-Badry, an associate professor of astronomy at the California Institute of Technology and an adjunct scientist at the Max Planck Institute for Astronomy in Germany.

The new study, which involved a team of co-authors from around the world, was published in The Open Journal for Astrophysics. Data from several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawaii, the La Silla Observatory in Chile, and the Whipple Observatory in Arizona, were used to follow up on Gaia's observations and learn more about the masses and orbits of the hidden neutron stars.

Although neutron stars have been found orbiting stars like our Sun before, those systems were more compact. With the small distance separating the two bodies, a neutron star (which is heavier than a Sun-like star) can steal mass from its partner. This mass transfer process causes the neutron star to shine brightly in X-rays or radio waves. In contrast, the neutron stars in the new study are much farther from their partners — the distance between Earth and the Sun is one to three times greater.

This animation depicts a binary star system in which a massive compact neutron star orbits a larger Sun-like star. The intense gravity of this high-density neutron star causes significant warping effects that distort the view of the sky around it, unlike what happens around more compact black holes.

This means that the newfound stellar corpses are too far away from their partners to steal material from them. Instead, they are dormant and dark. “These are the first neutron stars to be discovered solely through their gravitational influence,” says El-Badry.

The discovery was somewhat unexpected, since it is unclear how an exploded star ends up near a star like our Sun.

“We still don’t have a complete model of how these binary stars form,” explains El-Badry. “In principle, the neutron star progenitor would have grown huge and interacted with a solar-type star late in its evolution.” The huge star would have knocked over the small star, perhaps temporarily engulfing it. The neutron star progenitor would later explode in a supernova, which, according to models, would have decoupled the binaries, sending the neutron stars and solar-type stars hurtling off in opposite directions.

“The discovery of these new systems shows that at least some binary stars survive these cataclysmic processes, although models cannot yet fully explain how this happens,” he says.

Gaia was able to find unlikely companions because of their wide orbits and long periods (sun-like stars orbit neutron stars with periods ranging from six months to three years).

In this animation, Sun-like stars are colored green, and neutron stars (and their orbits) are colored purple.

“If the bodies are too close, the wobbles will be too small to detect,” says El-Badry. “With Gaia, we are more sensitive to wider orbits.” Gaia is also most sensitive to binaries that are relatively close by. Most of the newly discovered systems are within 3,000 light-years of Earth — a relatively small distance compared to, for example, the Milky Way galaxy’s diameter of 100,000 light-years.

The new observations also suggest how rare such pairs are. “We estimate that about one in a million solar-type stars orbits a neutron star in a wide orbit,” he says.

El-Badri is also interested in finding invisible, dormant black holes orbiting sun-like stars. Using Gaia data, he has discovered two such quiet black holes hidden in our galaxy. One of them, called Gaia BH1, is 1,600 light-years from Earth and is the closest known black hole.

“We don't know exactly how these binary black holes formed,” El-Badry says. “There are clear gaps in our models of binary star evolution. Finding more of these dark companions and comparing their population statistics to the predictions of different models will help us understand how they form.”